1. Field of the Invention
The present invention generally relates to an ophthalmologic apparatus capable of capturing an image of an ocular fundus, and in particular it relates to an ophthalmologic apparatus that includes an adaptive optical system.
2. Description of the Related Art
A scanning laser ophthalmoscope (SLO) is a well know example of an ophthalmologic imaging apparatus. To obtain an image of an ocular fundus with an SLO, an eye is irradiated with laser light in a two-dimensional pattern and the light reflected from the ocular fundus is received by an optical detector to form an image. Further, an imaging apparatus that utilizes interference of low coherence light has been developed as an ophthalmologic imaging apparatus. Such imaging apparatus that utilizes interference of low coherence light is called an optical coherence tomography (OCT) apparatus and is used, in particular, to capture a tomographic image of an ocular fundus or the vicinity thereof. Various OCT systems have been developed including a time domain (TD) OCT system and a spectral domain (SD) OCT system. In particular, in recent years, higher resolution has been achieved in an ophthalmologic imaging apparatus with an increase in the numerical aperture (NA) of an irradiating laser.
When an image of an ocular fundus is to be captured, the image needs to be captured through optical tissues within the eye, such as the cornea and the crystalline lens. Thus, as the resolution is increased, an aberration that occurs in the cornea and crystalline lens increasingly affects the quality of a captured image.
Accordingly, research has been carried out on adaptive optics (AO) SLO and AO-OCT, in which the function of the AO that measures the aberration of an eye and corrects the aberration is integrated into an optical system. For example, in an article entitled “Requirements for discrete actuator and segmented wavefront correctors for aberration compensation in two large populations of human eyes, N. Doble et al., Applied Optics, Vol. 46, No. 20, 10 Jul. 2007, discusses an example of AO-OCT. In AO-SLO or AO-OCT, changes in the wavefront of light incident on an eye is typically measured by a Shack-Hartmann wavefront sensor system. In the Shack-Hartmann wavefront sensor system, measurement light enters an eye, and the reflected light thereof is received by a charge-coupled device (CCD) camera through a microlens array, to thereby measure the wavefront of the reflected light. Since the optical tissues within the eye cause aberrations on the wavefront of the light, a reflective optical modulation device is driven to correct the aberrations of the measured wavefront, and an image of the ocular fundus is captured through the reflective optical modulation device. Thus, AO-SLO or AO-OCT can capture the image at high resolution.
There are several types of reflective optical modulation devices including a variable shape mirror and a reflective liquid crystal device. Since the reflective liquid crystal device modulates polarization directions and thus requires two optical elements, which leads to an increase in size of the optical system. Furthermore, because of its high wavelength dependence, the reflective liquid crystal device is not suitable for observation at multiple wavelengths, which is often employed in the observation of an ocular fundus. Thus, a variable shape mirror is preferably used as the reflective optical modulation device.
The variable shape mirror corrects the wavefront by deforming the mirror shape and thus by generating an optical path length difference within the effective diameter in the optical axis direction. Here, a difference in coordinates, in the optical axis direction, of two positions on the mirror surface within the effective diameter is referred to as a displacement amount. Since a mirror is a reflective element, a required maximum displacement amount is half a maximum optical path length to be corrected.
There are various types of variable shape mirrors. Specifically, there is a mirror having a continuous mirror surface that is caused to deform by a plurality of actuators. There is also a segmented mirror that is formed of multiple segments, and the segments are independently driven by respective actuators to be translated in the optical axis direction. There is another type of segmented mirror that is formed of multiple segments, and the segments can be independently driven by respective actuators to be translated in the optical axis direction and tilted about two axes.
Japanese Patent Application Laid-Open No. 2005-224327 discusses a technique in which reflected light from an ocular fundus that includes an aberration is corrected by using at least two reflective optical modulation devices in order to increase the correction amount.
Japanese Patent Application Laid-Open No. 2007-21044 discusses a finding that correction of an aberration of a light flux incident on a model eye using an 85-segmented reflective optical modulation device having a diameter of 12 mm and a maximum displacement amount of 16 μm has resulted in a residual aberration RMS of 0.093 μm. However, Japanese Patent Application Laid-Open No. 2007-21044 does not discuss the number of mirror segments within the effective diameter.
N. Doble et al., Applied Optics, Vol. 46, No. 20, 10 Jul. 2007 discusses a relationship between a Strehl ratio, which indicates the accuracy of the wavefront correction, and the number of actuators for driving the reflective optical modulation device, and a displacement amount of each actuator required for correcting an aberration of a human eye.
Early detection of a progressive disease in an ocular fundus is important. Thus, there is a need for an observation apparatus that is capable of detailed observation at the visual cell level. Visual cells include two types of cells, namely a cone cell having a size of approximately 2 μm to 5 μm and distributed around the macular area, and a rod cell having a size of approximately 2 μm and primarily distributed outside the macular area. A cone cell has a small size of approximately 2 μm in the central fovea in the macular area. Accordingly, detecting an ocular fundus disease at an early stage requires the resolution of approximately 2 μm.
As stated above, an ophthalmologic apparatus into which the function of an adaptive optical system is integrated in order to achieve higher resolution has been developed. However, since eyes to be examined have individual differences in the optical aberration, there has been an issue that a clear image of the visual cells cannot be obtained depending on an eye to be examined.
Although Japanese Patent Application Laid-Open No. 2005-224327 discusses a technique for obtaining a desired correction amount by us ing a plurality of variable shape mirrors, such a technique makes it difficult to obtain a compact optical system.
Although Japanese Patent Application Laid-Open No. 2007-21044 discusses a finding that the aberration has been corrected to the residual aberration RMS of 0.093 μm by using a single variable shape mirror, this residual aberration amount cannot bring the Strehl ratio to 0.8 or less, and thus sufficient optical performance cannot be secured.